This study will use positron emission tomography (PET) imaging and cerebralspinal fluid (CSF) studies to probe for possible MDMA-induced 5-HT neurotoxicity in humans. The long-term goal of the project is to better delineate the public health risks and consequences of recreational use of MDMA, a popular recreational drug. The overall goal of the project is to use PET imaging in conjunction with studies of CSF serotonin metabolites to ascertain whether humans previously exposed to high doses of MDMA have sustained serotonergic neuronal injury.